Whether it’s a bird that starts its day at first light, a fish that migrates to the ocean surface at night, or a fruit fly that is mainly active in dusk or dawn, virtually all living beings on this planet follow a daily pattern of activity. This is a consequence of the earth’s rotation, generating predictable daily fluctuations of light and temperature as well as food availability that most organisms are exposed to. It is not surprising that during evolution a wide range of organism from cyanobacteria to humans have adapted to this 24 h rhythm by developing an endogenous timing system. Such biological clocks allow them to synchronize their metabolism, physiology and behavior with the external environment. Characteristically, in the absence of external time cues these endogenous clocks tick with a period of close to but not exactly 24 h, hence they are called circadian clocks (Latin: “circa dies”: about a day). While the adaptive value of circadian rhythms is not fully understood, it is remarkably conserved across species, and disruptions to an animal’s circadian cycle produce stress. In humans, such disruptions are associated not only with insomnia, depression and ‘jet lag’-like symptoms, but importantly also with an increased risk for metabolic syndrome and cancer.
Circadian clocks have been identified essentially in every light-responsive organism ranging from cyanobacteria to humans and are conceptually based upon a similar molecular makeup: transcriptional/translational auto regulatory feedback loops involve rhythmic clock gene expression and – increasingly recognized – posttranslational events generating ~24 h rhythms on a molecular level . They are believed to have evolved in parallel with the geological history of the earth, and have since been fine-tuned under selection pressures imposed by cyclic factors in the environment. Circadian clocks may not only be critical for synchronization to the solar day but also for the control of seasonal events, such as reproduction and hibernation. The most widely used environmental stimulus that synchronizes internal clocks with the outside world is light (photoperiod).
Life evolved in the sea, an environment that is governed by a multitude of environmental cycles including not only the daily light/dark cycle, but also shorter cycles such as tides, or longer cycles such as lunar cycles or annual seasons. Endogenous rhythmic and synchronized behaviors with periodicities ranging from circadian (Daily Vertical Migration), to circa lunar (molt-spawning cycle), to circa annual (sexual maturation and reproduction) have been described in a wide range of marine organisms . However, our current molecular understanding of biological rhythms and clocks is largely restricted to circadian and seasonal rhythms in land model species such as the fruit fly, the mouse or the thale cress. In marine organisms in general, little is known about the principles of endogenous clocks and how these clocks interact with environmental cycles. This is particularly true for high latitude pelagic organisms. Endogenous biological rhythms in polar pelagic organisms and their associated molecular mechanisms have yet not been characterized, presumably due to the inaccessibility to these regions and problems with rearing these organisms for long term experiments in the laboratory under adequate conditions.
The polar pelagic environment is characterized by extreme seasonal changes in environmental factors such as day length, light intensity, sea ice extent and food availability. Biological timing of its residents that guarantees regulation of physiology and behaviour in reaction to annual fluctuations of biologically significant factors is thus of particular advantage. Not surprisingly, polar pelagic organisms have evolved endogenous rhythmic physiological and behavioral functions which are synchronized with these cyclic changes [e.g.3-5]. The mechanisms, however, leading to these rhythms are far from clear.
Moreover, the polar and sub-polar latitudes, comprising the fastest warming regions on the planet with profound impacts on the marine environment e.g. sea ice decline, temperature rise and ocean acidification [6-8]. Future predictions based on climate models foresee no end of amplified warming at the poles, suggesting ecosystem shifts and reorganizations in the pelagic environment with profound consequences for trophic interactions and biogeochemical cycles.
To predict effects on ecosystems at species and community levels, it is therefore of paramount importance to understand the basic principles of how the life cycle of key species, which in turn have a large impact on the environment, is synchronized with their seasonal environment. In this respect it is of fundamental scientific interest to understand the molecular basis of biological rhythms and clocks in polar pelagic organisms that have a central importance in polar pelagic food webs and to evaluate how these rhythms and their molecular mechanisms will be affected by climate induced environmental changes. A question of central importance is if these organisms can adapt fast enough to keep up with their changing environment. Endogenous clocks of land- based species are protected from changes in temperature, nutrition and pH, within physiologically permissible limits. The range of conditions in which the clocks are operating in polar marine organisms is not understood. It is also not known which physiological and behavioral consequences might emerge when the daily and seasonal timing systems of polar organisms exceed their normal limits. The ongoing environmental alterations might desynchronize previously matched interactions between the endogenous seasonal rhythms of key species (e.g. metabolic regulation, sexual maturation, and lipid accumulation) and their environment (e.g. seasonal sea ice dynamic, spring diatom blooms) which have evolved over millions of years.
Understanding the fundamentals and consequences of biological timing of key polar pelagic organisms under current and likely future conditions is crucial for allowing predictions of the consequences of future environmental changes on the biodiversity and productivity of Polar Regions. This is of global importance as the polar oceans are highly productive and hold a significant proportion of the world’s marine biomass.